In a typical system comprised of digital components, a bus connects the components together and is the conduit by which each component communicates information to another component. A bus usually consists of several lines or wires, each carrying a dedicated signal such as power, reference (ground), clock, synchronizing, and data signals. Data is communicated over one or more wires according to a protocol or specification which provides a set of rules by which components are permitted access to and use of the bus. In a serial bus, data can be transferred serially in a unidirectional or bi-directional fashion. In the case of a unidirectional serial bus, only one wire is needed to transmit or receive data between two components connected to the bus. However, in the case of a bi-directional serial bus, typically two wires are needed, one to transmit and the other to receive data between two components connected to the bus.
Several serial busses exist in the personal computer environment. For example, the Universal Serial Bus (“USB”) and IEEE1394 (“Firewire”) busses are examples of serial busses that operate in a high-power environment. Another bus that is used in PC audio systems is the AC97 link. However, none of these busses is well-suited for low-power, low-voltage environments because of their power demands. In addition, current communications busses take few precautions to ensure the integrity of signals in noisy environments. Certain low-power devices, such as hearing instruments and cellular phones, are highly susceptible to electromagnetic interference. Moreover, low-power devices tend to be small, and precious space can be saved by reducing the number of connections. Thus, many low-power devices tend to use serial busses to reduce the total number of wires.
Furthermore, in order to define the logic levels on a bus such as when bus owners are absent or there are no devices driving the bus, a pull-up or a pull-down resistor is traditionally used. However, this approach is problematic for in low-power environments that require short time constants. A high value pull-up or pull-down resistor results in a low power consumption, but long time constants (i.e., the bus will be slow). A low value pull-up or pull down resistor results in a short time constant, but unfortunately also in a relatively high current consumption.
Therefore, it would be advantageous to provide a low-voltage, low-power bus that reduces the number of wires without reducing the number of signals carried over the bus and that has well-defined logic levels without compromising bus speed. The present invention provides this and other advantages.